gnu-maintenance: Produce mirror:// URIs in latest-html-release.

gnu-maintenance: Produce mirror:// URIs in latest-html-release.

Partially fixes <https://issues.guix.gnu.org/57477>.

I'm not aware of a package using both latest-html-release and mirrors, so it
has not been completely tested.  However, updating "yt-dlp" appears to work
(except for git-fetch not being supported yet).  The expression for the
signature-urls field had to be tweaked to not call uri-mirror-rewrite on #false.

* guix/gnu-maintenance.scm (latest-html-release)[url->research]{urls,signature-urls}:
Call uri-mirror-rewrite on the URLs.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>